As will be appreciated herein below, except as otherwise indicated, all aluminium alloy designations refer to the Aluminum Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2008.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
Age-hardenable wrought aluminium alloys are used amongst others for aerospace applications because of their combination of strength, corrosion resistance, and damage tolerance properties. Plates from these products are commonly produced by a process involving casting, forming by means of rolling and/or forging, solution heat treating, quenching of the solution heat treated product, and ageing of the quenched product. The quenching process also leaves high residual stress, which cannot be thermally relieved while maintaining the alloy product's favourable mechanical properties. Therefore, the stresses are relieved or at least reduced by applying a uniform plastic strain, which for rolled plate involves uniaxially stretching in the rolling direction, and when applied on an industrial scale typically from about 1.5% to 3% strain, and followed by ageing which consequently carries the Tx51 temper designation.
In an alternative process the rolled plate is compressed in a forging operation commonly by overlapping steps followed by ageing and consequently carries the Tx52 temper designation. Such a forging operation by compressing of thick plates or blocks is for example disclosed in patent document WO-2004/053180-A2.
The article “Relief of Residual Stresses in a High-Strength Aluminum Alloy by Cold Working” by Y. Altschuler et. al., published in “Mechanical Relaxation of Residual Stresses”, ASTM STP 993, L. Mordfin, Ed., American Society for Testing and Materials, Philadelphia, 1988, pp. 19-29, concerns the relief of residual stresses derived from the rapid quenching of 7075 aluminium in the form of sheet of 31.8 mm. It was found that mechanical stress relief in tension is to be preferred to that in compression.
U.S. Pat. Nos. 6,159,315 and 6,406,567 disclose methods of stress relieving solution heat-treated and quenched aluminium alloy plates that includes a combination of a stress-relieving cold mechanical stretch and a stress-relieving cold-compression, the cold stretch being performed in the length direction, and the cold compression being performed in the thickness direction.
U.S. Pat. No. 6,569,542 discloses a structural element made from a 2xxx-series alloy having a thickness of at least 10 mm and treated by solution heat treating, quenching, permanent tension to more than 1.5% permanent deformation by means of stretching, and ageing.
U.S. Pat. No. 6,077,363 discloses an AlCuMg sheet product, said sheet having reduced deflection after machining, and whereby the sheet product has been quenched and stretched.
Aluminium sheet products or thin gauge plate products (less than about 20 mm in thickness) may be stretched or leveled by rolls to improve metal flatness and it might result also in a small reduction of residual stress. Levelling by rolls consists of passing the sheet product between two or more series of parallel rolls placed alternately below and above the sheet, the rolls being nested. The sheet product is then alternately deflected in one direction and then in the other direction to obtain plastic deformation. For thicker gauge products (more than about 20 mm in thickness) there are no industrial machines that can accept these kinds of products without adversely affecting the engineering properties of the plate product. Furthermore, the deformation by means of roll levelling is not sufficiently controllable to achieve reproducible characteristics in relaxation of residual stress after quenching, which is easier with a stretcher than with a roll levelling machine, at least in the case of thicker gauge sheet products.
For conventional industrial scale stretching of plate products (about 20 mm or more), the ends are trapped between two jaws and then a permanent controlled elongation is applied. The stretching machine comprises a fixed head with jaws and a mobile head comprising the other jaws. If the cross-section of the plate product is large (e.g. very thick or very wide or both) the strength of the stretching machine, and in particular the clamping force of the jaws, may be insufficient to achieve the desired stretching degree.